Digital frequency synthesizers are replacing oscillators in a number of important applications such as frequency generating subsystems, test equipment and communication systems. In particular, many communication systems use complex modulation schemes which are often more easily implemented using digital frequency synthesis.
Battery-powered communication systems using spread spectrum technology are one category of applications that benefit from digital frequency synthesis. This category includes cellular telephone systems and ground terminal communication systems. Such applications may use frequency hopping techniques to spread a signal over the channel bandwidth. These applications will benefit from a low-power, wideband frequency synthesizer with high spectral purity. Current technology has failed to achieve those goals.
One existing device is the direct digital synthesizer (DDS). Conventionally, a high-resolution DDS generates wideband analog signals using a high-speed digital clock. For example, frequency responses up to 400 MHz have been demonstrated using a 1 GHz clock. However, due to phase truncation effects, amplitude quantization effects, and digital-to-analog converter (DAC) nonlinearities and glitches, a large number of high level spurious signals are present in the output frequency band. These spurious signals increase with the clock frequency.
In addition to spectral purity problems, conventional DDSs suffer from hardware constraints, Because of the high clock frequencies, conventional high-speed DDSs are fabricated in Gallium Arsenide (GaAs). Furthermore, the conventional DDS dissipates higher power when driven at high clock frequencies.
Serrodyne modulators have been used to modulate high frequency analog signals by lower frequency digital signals. For example, this technique is used to generate frequency chirps for radar systems. The resulting frequency translation is relatively narrow band, typically less than one octave. In addition, the resulting signal has mediocre near carrier, spurious performance, typically -15 to -30 dBc. The use of analog devices to generate the phase shifts further limits performance.
One such serrodyne modulator is discussed by Smith et al. in U.S. Pat. No. 5,063,361, which discusses a direct digital synthesizer having a phase accumulator for changing the phase of an RF signal. The device uses dual shifter channels in a ping-pong arrangement. Channel switching is done using make-before-break RF switches, which are switched on every clock cycle alternate phase shifting of the RF signal in alternate channels. The DDS is implemented as a digital monolithic microwave integrated circuit (MMIC).
Phase modulation has been discussed by Hiramatsu in European Patent Publication 0 419 680 as frequency-modulating a digital signal by a carrier frequency. In particular, an analog modulation signal is digitally processed to be FM modulated. The output signal is centered at the carrier frequency.
Doughty in U.S. Pat. No. 3,749,937 discusses an electronic musical tone modifier. An analog audio frequency is FM modulated by a sub-audible signal. The modifier can include a digital or an analog shift register to perform the frequency modulation. The output signal is an analog audio signal.